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CMOS Sensors Get UV-Transparent Coating

Photonics.comFeb 2011
DUISBURG, Germany, Feb. 28, 2011 — Image sensors are partially color-blind because their silicon nitride coatings prevent UV light from passing through, making them unsuitable for spectroscopy. Now a new production process, discovered by scientists at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS, makes the coatings transparent — and the sensors suitable for special applications.
UV-transparent coating for image sensors. (Image: Fraunhofer IMS)
Standard devices in multimedia electronics for a long time, CMOS image sensors are making rapid inroads in high performance applications: They are no longer only used in cell phones and digital cameras. The automotive industry, for instance, has discovered the potential of optical semiconductor chips and is increasingly using them in driver-assistance systems — from parking aids and lane detection to blind-spot warning devices. In special applications, however, the sensors that convert light into electrical signals must cope with difficult operating conditions; for example, high temperatures and moisture.

For this reason, CMOS devices are covered with a silicon nitride coating. This chemical compound forms hard layers that protect the sensor from mechanical influences and the penetration of moisture and other impurities. The protective coating is applied to the sensor in the final stage of CMOS semiconductor production. The process is called passivation, and is an industry requirement. Unfortunately, up to now passivation has entailed a problem: the silicon nitride coating limits the range of optical applications because it is impermeable to light in the UV and blue spectral ranges. CMOS sensors for high-performance applications — used in special cameras — are, therefore, partially color-blind.

“We’ve developed a new process step,” said Fraunhofer IMS’ Werner Brockherde, “that allows us to produce a protective coating with the same properties but which is permeable to blue and UV light.” The trick is to increase the proportion of nitrogen in the coating. “This reduces the absorption of shortwave light."

In simplified terms, the new coating material will absorb less light of an energy higher than blue light, which means the sensor becomes more sensitive at the blue and UV range.

“This makes CMOS image sensors suitable for use in wavelength ranges down to 200 nm,” Brockherde said. “With standard passivation the limit was about 450 nm.” To change the structure of the silicon nitride for the coating, the Fraunhofer IMS research scientists had to fine-tune the deposition parameters such as pressure and temperature.

With this process development, the experts have expanded the range of applications for CMOS image technology. This could revolutionize UV spectroscopic methods, which are used in laboratories around the world, significantly improving their accuracy. Likewise, CMOS image sensors stand to take up a new role in professional microscopy; for example, in fluorescence microscopes, providing scientists with images of even greater detail.